The present invention relates to an electronic overload relay. In particular, the present invention relates to the source of power of electronic control circuit in an overload relay.
It is known to provide the control circuitry for overload relays and circuit breaker power which is derived from the current transformers used to monitor the current in the power conductors associated with the circuit breaker or overload relay. To provide sufficient power for the operation of a control circuit which has sufficient capacity to properly control an overload relay, the current transformers used for monitoring the power conductors must be increased in size relative to current transformers which would be used solely for monitoring the current in the power conductors. This increase in size increases the cost of the current transformers and also increases the size of the overload relay since the current transformers must fit within the housing of the overload relay.
In addition to the increase in size required when current transformers used for monitoring the current in phase conductors are also used to power the control circuit, the signals produced by the current transformers are typically affected adversely. More specifically, when power is drawn from the current transformers, the signals produced by the current transformers do not accurately produce signals representative of the current flowing in the associated power conductors.
Accordingly, it would be advantageous to provide power to the control circuit of an overload relay from a source other than the current transformers monitoring the current flow in the associated power conductors without modifying industry standard wiring practice established with conventional thermal overload delays.